1. Field of the Invention
The present invention relates to a capacitive pressure sensor having a diaphragm.
2. Description of the Related Art
In recent years, a capacitive pressure sensor has been widely used in that it is small and a structure thereof is simple. For example, the capacitive pressure sensor has been adopted as a means for monitoring the tire pressure of an automobile.
A conventional capacitive pressure sensor is disclosed in Japanese Patent No. 2772111. As shown in FIG. 1 of Japanese Patent No. 2772111, the capacitive pressure sensor is configured such that a glass substrate is bonded to a silicon substrate having a pressure sensitive diaphragm portion, a reference pressure chamber is provided between the silicon substrate and the glass substrate in the vicinity of a location where the pressure sensitive diaphragm portion is formed, and a first electrode is formed at a side of the reference pressure chamber facing the glass substrate. Further, in the vicinity of the reference pressure chamber, the glass substrate is provided with a first opening portion and the first opening portion is provided with a first lead portion. Furthermore, on a surface of the glass substrate facing the reference pressure chamber, a cover plate composed of a silicon plate is provided to abut the first lead portion. In addition, a part of the first electrode is connected to the cover plate.
In the conventional capacitive pressure sensor described above, since the first electrode is connected to the first lead portion of the glass substrate not facing the reference pressure chamber through the cover plate, the cover plate and the first lead portion can be used as lead wiring lines of the first electrode.
However, in the capacitive pressure sensor disclosed in Japanese Patent No. 2772111, it is necessary to dispose the cover plate in the vicinity of the reference pressure chamber so as to lead the first electrode. Since the cover plate is disposed to be face the silicon substrate having the pressure sensitive diaphragm portion, a static capacitance is generated between the cover plate and the silicon substrate to become a parasitic capacitance. As a result, there is a possibility that the parasitic capacitance will cause the pressure sensitivity of the capacitive pressure sensor to be lowered. Accordingly, as shown in FIG. 1 in Japanese Patent No. 2772111, in the capacitive pressure sensor disclosed in Japanese Patent No. 2772111, a reference-pressure-chamber-side surface of the silicon substrate facing the cover plate is recessed farther from the diaphragm portion, so that the distance between the silicon substrate and the cover plate becomes large. Thereby, the parasitic capacitance is reduced, which prevents the pressure sensitivity from being lowered.
In the capacitive pressure sensor, it is necessary to secure a region where the lead wiring lines are to be formed in the vicinity of the reference pressure chamber so as to provide the cover plate and the first lead portion serving as the lead wiring lines of the first electrode. Accordingly, there has been a problem in that the pressure sensor cannot be made small. In addition, in order to recess the part of the silicon substrate corresponding to the cover plate for the purpose of reducing the parasitic capacitance, the thickness of the silicon substrate should be relatively large. For this reason, there has been a problem in that it is difficult to make the pressure sensor thin.
Further, in order to prepare the lead wiring lines, complicated processes are needed in which the first opening portion is provided on one surface side of the glass substrate, the first lead portion is formed in the opening portion by an etching process, and the cover plate is formed on the other surface of the glass substrate. Accordingly, the structure of the sensor becomes complicated. In particular, since a structure in which the reference pressure chamber is sealed by the cover plate is adopted, there is a possibility that the reference pressure chamber will not be reliably sealed.